;*******************************************************************
;                           avrwalk.asm
;
; This code walks LEDs backwards and forwards on port b. 
; Connect leds from port b (via current limiting Rs) to +5V.
; This code is also compatible with STK200.
;*******************************************************************

;***** Specify Device
.device AT90S1200

;***** I/O Register Definitions
.equ	PORTB	=$18 		; port b data register
.equ	DDRB	=$17 		; port b data direction register
.equ	PINB	=$16 		; port b input pins register

;
;
;
; can only use R16 to R31 for constants
; and logic instructions such as SBCI, SUBI, CPI, ANDI and ORI
; and most importantly LDI!

.def tempport	=r16	
.def count1	=r17		; and logic instructions
.def count2	=r18

.def	XL	=r26
.def	XH	=r27
.def	YL	=r28
.def	YH	=r29
.def	ZL	=r30
.def	ZH	=r31

;******************************************************************
.cseg					;CODE segment
.org 0
	rjmp	init			; origin

.org 4
init:   
	ldi	tempport,$FF
	out  	DDRB,tempport   	; set portb as outputs
	ser	tempport
	out  	PORTB,tempport		; set portb levels all high

start:
	ldi	tempport,$FE    	; set tempport bit 0 low
	out	PORTB,tempport		; write to port

rot_L:
	rcall 	wait			; wait for a bit
	sec				; set the carry bit
	rol	tempport		; rotate left tempport
	out	PORTB,tempport		; write to port 
	sbrc	tempport,7		; skip next line if top bit clr
	rjmp	rot_L

rot_R:
	rcall 	wait
	sec				; set the carry bit	
	ror	tempport		; rotate right tempport
	out	PORTB,tempport		; write to port 
	sbrc	tempport,0		; skip next line if bottom bit clr
	rjmp	rot_R
	
	rjmp 	rot_L			; do it all again	

; ----------------------------
; wait subroutine 
; ----------------------------
wait:    
	ldi	count1,200		; load count1 with decimal 200

d1:
	ldi	count2,200		; load count2 with decimal 200

d2:
	dec	count2			; decrement count2
	brne	d2			; loop if not zero		
	dec  	count1			; decrement count1 if count2 is zero
	brne 	d1			; do inside loop again if count2 nz
	ret
; ----------------------------